Glossary

The time required for the activity of a given radioactive substance to decrease to half of its initial value due to radioactive decay. The half-life is a characteristic property of each radioactive element and is independent of its amount or condition.

A target protected against the blast, heat, and radiation effects of nuclear weapons of specific yields. Hardening is usually measured by the number of psi of blast overpressure which a target can withstand.

Heavy water

Water in which the hydrogen is composed of over 99 percent deuterium atoms. The neutron in the deuterium nucleus allows this type of water to slow, or moderate, neutrons from fissioning uranium, permitting a sustained chain-reaction in reactors using natural uranium as fuel.

The height above the earth's surface at which a bomb is detonated in the air. The optimum height of burst for a particular target is that at which it is estimated a weapon of a specified energy yield will produce a certain desired effect over the maximum possible area.

High-level waste

Highly radioactive waste material from the reprocessing of spent nuclear fuel -- including liquid waste produced directly in reprocessing and any solid waste derived from the liquid -- that contains a combination of transuranic waste and fission products in concentrations requiring permanent isolation.

High Altitude Burst

This is defined, somewhat arbitrarily, as a detonation at an altitude over 100,000 feet. Above this level the distribution of the energy of the explosion between blast and thermal radiation changes appreciably with increasing altitude due to changes in the fireball phenomena.

Highly enriched uranium (HEU)

Uranium in which the naturally occurring U235 isotope - 0.7 percent in natural uranium, 99.3 percent U238 - is increased to 20 percent U235 or above, but usually to 90 percent or more. HEU is used in nuclear weapons, and in some types of research and submarine propulsion reactors.

Hiroshima

The first use in warfare of a nuclear weapon occurred on August 6, 1945, at 8:16:02 a.m. over Hiroshima, Japan. In an instant 80,000 to 140,000 people were killed and 100,000 more were seriously injured.

Hiroshima ceased to exist as a functioning city. The bomb exploded almost directly over the center of the city. Two square miles of the city were completely leveled by the bomb.

Lead-shielded boxes with protected view plates and remote manipulators used for working with radioactive materials. Some hot cells are suitable for reprocessing small amounts of spent reactor fuel to obtain plutonium.

Hydrogen Bomb

The hydrogen bomb, or H-bomb, is a nuclear weapon in which light atomic nuclei of hydrogen are joined together in an uncontrolled nuclear fusion reaction to release tremendous amounts of energy. The hydrogen bomb is about a thousand times as powerful as the atomic bomb, or A-bomb, which produces a nuclear fission explosion about a million times more powerful than comparably sized bombs using conventional high explosives such as TNT.

A naturally-occurring element required in small amounts for the normal working of the thyroid. A radioactive isotope is created in nuclear explosions and in nuclear reactors.

Implosion Weapon

A device in which a quantity of fissionable material, less than a critical mass, has its volume suddenly decreased by compression, so that it becomes supercritical and an explosion can take place. The compression is achieved by means of a spherical arrangement of specially fabricated shapes of ordinary high explosive which produce an inwardly-directed implosion wave, the fissionable material being at the center of the sphere.

A weapon that uses a simple, untested design to attempt to create a nuclear explosion.

Induced Radioactivity

Radioactivity produced in certain materials as a result of nuclear reactions, particularly the capture of neutrons, which are accompanied by the formation of unstable (radioactive) nuclei. In a nuclear explosion, neutrons can induce radioactivity in the weapon materials, as well as in the surroundings (e.g., by interaction with nitrogen in the air and with sodium, manganese, aluminum, and silicon in soil and sea water).

Initial Nuclear Radiation

Nuclear radiation (essentially neutrons and gamma rays) emitted from the fireball and the cloud column during the first minute after a nuclear (or atomic) explosion. The time limit of one minute is set, somewhat arbitrarily, as that required for the source of part of the radiations (fission products, etc., in the radioactive cloud) to attain such a height that only insignificant amounts of radiation reach the earth's surface.

Initiator

An initiator is a device that produces neutrons at just the right instant, when the assembly process has reached the stage at which the fissile material has supercritical mass. This allows some of the early stages in the chain reaction to be bypassed. As the protective shell collapses, the alpha particles combine with the beryllium powder to form huge amounts of neutrons for the chain reaction.

A ballistic missile with a range of 3,400 miles or more. Conventionally, the term ICBM is used only for land-based systems, in order to differentiate them from submarine-launched ballistic missiles (SLBMs), which can also be intercontinental range.

Intermediate Range Nuclear Forces (INF)

Missiles with ranges of 500 to 5,500 kilometers (300-3,400 miles)

Intermediate-Range Nuclear Forces (INF) Treaty

This treaty between the United States and the former Soviet Union. It aimed to eliminate and ban all ground-launched ballistic and cruise missiles with a range of 300-3,400 miles (500-5,500 kilometers). By May 1991, all intermediate-range and shorter-range missiles, launchers, related supportwere eliminated.

International Atomic Energy Agency (IAEA)

The IAEA serves as an intergovernmental forum for scientific and technical co-operation in the peaceful use of nuclear technology. It was established as an autonomous organization on July 29, 1957, seeks to promote the peaceful use of nuclear energy and to inhibit its use for military purposes.

Inverse Square Law

The law which states that when radiation (thermal or nuclear) from a point source is emitted uniformly in all directions, the amount received per unit area at any given distance from the source, assuming no absorption, is inversely proportional to the square of that distance.

Ionize

To split off one or more electrons from an atom, thus leaving it with a positive electric charge. The electrons usually attach to one of the atoms or molecules, giving them a negative charge.

A gamma-ray emitting radioisotope used for gamma- radiography. The half-life is 73.83 days.

Irradiate

To expose to some form of radiation.

Isotope

The term isotope defines atoms that have the same number of protons but a different number of neutrons; that is, they are atoms of the same element that have different masses. Their atomic number (proton number) is the same, but their mass numbers (the total number of protons and neutrons in the nucleus) vary. Forms of the same element having identical chemical properties but differing in their atomic masses and in their nuclear properties.

Kiloton

This is approximately the amount of energy that would be released by the explosion of 1,000 tons of TNT.